Spotty Lighting Showing From Sun Light
A refraction shader with an IOR of 1 should do very little, but I think refraction shader by itself messes with light paths just a little.  I also seem to remember Transparent BSDF working better with an Add Shader.  From the reference:
Transparent BSDF.
The Transparent BSDF is used to add transparency without refraction, passing straight through the surface, as if there were no geometry there. Useful with alpha maps, for example. This shader affects light paths somewhat differently than other BSDFs. Note that only pure white transparent shaders are completely transparent.
I would start out with a reduced node setup, and you can add to it and verify the effects.  You could go to the principled shader if you feel more comfortable there.

If you use the color picker you'll see the light spot on the floor and the edge of the plane both top out around .68 Value, so I don't think this is doing anything too inaccurate.  The light hitting the camera from outside the window is around .88.


This should work fine for thin panes of glass.  For more complex pieces this may produce lighting that is too bright, such as in the following example where the transparent bsdf is set to .6 (.9 is pretty much unreasonable):

https://blenderartists.org/t/how-do-i-make-glass-cast-transparent-shadows-in-cycles/650692/5
Here's a thread from blenderartists where user theoldghost suggests the node group used in the following image where the transparent bsdf is set to .9: